1. Field of the Invention
This invention relates to a control system for a cylinder cut-off internal combustion engine of a vehicle wherein a fuel supply to some of the cylinders is cut off.
2. Description of the Related Art
In a conventional multi-cylinder internal combustion engine having a plurality of cylinders, it has been proposed to improve fuel consumption by switching engine operation, based on the engine load, between full-cylinder operation during which all of the cylinders are supplied with fuel to be operative and cut-off-cylinder operation during which the fuel supply to some of the cylinders are cut-off or stopped to be non-operative. In this type of engine, since shock is occasionally generated due to the fluctuation of torque during engine operation changeover, it has also been proposed to eliminate shock by adjusting throttle opening during a transitional period of changeover, as taught in Japanese Laid-Open Patent Application No. Hei 10 (1998)-103097, for example.
It is also known to perform running control to control the vehicle to cruise at a desired vehicle velocity set by the operator. A similar preceding vehicle follow-up control (so-called “adaptive cruise control”) is known wherein the vehicle is controlled to detect the distance between itself and a preceding vehicle using a radar or the like, and to travel while maintaining a desired inter-vehicle distance between itself and the preceding vehicle. In these types of control, the vehicle velocity or inter-vehicle distance when the operator manipulates a corresponding setting switch is stored as a desired velocity or distance, and throttle opening is adjusted via an actuator such that the vehicle runs at the stored desired velocity or the vehicle runs at a desired velocity necessary for maintaining the desired inter-vehicle distance between itself and the preceding vehicle, as disclosed, e.g., in Japanese Laid-Open Patent Application No. Hei 9 (1997)-290665.
Moreover, it has also been proposed to raise a threshold value in a cylinder cut-off engine, when cruise control is in progress, such that engine operation is likely to be switched to cut-off-cylinder operation, as disclosed, e.g., in Japanese Patent Publication Sho 58(1983)-17338.
In such a cylinder cut-off engine, engine operation is switched between full-cylinder operation and cut-off-cylinder operation based on at least throttle opening operated in response to the operator's accelerator manipulation, engine speed and manifold absolute pressure, etc. Generally, in cruise control, in response to a drop in vehicle velocity, the throttle valve is moved by an actuator more finely and more quickly than done by the operator's manipulation.
Accordingly, when vehicle velocity drops after being switched from full-cylinder operation to cut-off-cylinder operation, or when vehicle velocity drops during cut-off-cylinder operation due to change in road gradient (slope), throttle opening is quickly controlled. However, since engine torque is normally insufficient at that situation, the vehicle velocity does not rise immediately and the throttle valve is liable to be opened excessively. This excessive opening of the throttle valve is a mere temporal event. Nevertheless, when engine operation switchover is determined based on the parameters including the throttle opening, this sometimes makes it difficult to switch engine operation to cut-off-cylinder operation to improve fuel consumption.
Moreover, in the case that engine operation switchover is determined based on the parameters including the throttle opening, when cruise control is in progress, engine operation is switched to cut-off-cylinder operation in response to a momentary throttle opening closing control, but is returned to full-cylinder operation if the throttle valve is again controlled to an opening direction, thereby occasionally producing a control hunting. In the technology mentioned in '338, it is proposed to raise a threshold value in a cylinder cut-off engine, when cruise control is in progress, such that engine operation is likely to be switched to cut-off-cylinder operation. However, since this technology only proposes to raise the threshold value, it can not always cope with running condition where the vehicle runs actually and hence, can not avoid frequent engine operation changeover.